The present invention relates to a macroporous support having a permeability gradient along the path of flow of a fluid to be treated, and to a method for producing it. The invention notably relates to such a macroporous support in sintered ceramic, sintered glass, sintered metal, or carbon material, provided with one or several longitudinal parallel channels, the surface of said channels being covered with one or several filtering layers in a sintered ceramic or organic material, in which a liquid to be purified or concentrated, or generally speaking a fluid to be treated, circulates. The assembly comprising the macroporous support and the filtering layer is referred to below as the membrane.
In such a device, the fluid to be treated enters an inlet chamber at one inlet end of the (macro)porous support or block and flows through the channels to the outlet end towards an outlet chamber, a portion of the liquid to be treated, or permeate, passing radially through the layer and the macroporous support, and being collected in a permeate-side outlet chamber.
According to the cross-flow filtration principle, the liquid to be treated circulates along the channel(s), and this flow leads to a pressure drop between the inlet and outlet of said channels. This pressure drop depends on a set of parameters such as, for example, the speed of the liquid to be treated or purified in the channel, its viscosity, as well as the hydraulic diameter of the channel. This decreasing variation in fluid pressure along the channel(s) modifies the transverse flow of the permeate passing through the filtering layer and then the macroporous body.
This results in a decreasing transmembrane pressure, i.e. in the difference between the pressure at a point in the channel and the pressure in the permeate chamber, in the direction of circulation of the liquid in the channel(s). This decreasing variation can affect the performance of the filtration device, by, for example, reducing the permeate throughput, and by modifying, for example, the retention threshold, and can also lead to different filtration conditions prevailing along the channel(s).
For example, in a conventional membrane having 4 mm diameter channels, the inlet pressure to the channels is 3.8 bar, the channel outlet pressure is 2 bar, while the pressure inside the permeate outlet chamber is constant, for example 1.5 bar. Thus, the transmembrane pressure varies along the membrane between 2.3 and 0.5 bar.
With such a conventional membrane, the set of dimensional parameters, associated with the geometry of the filtering element, hydraulic parameters associated with the liquid to be treated and with the operating conditions, do not make it possible to fully optimize the filtering operation as it is impossible to provide the optimum transmembrane pressure at all points along the membrane.
State of the Art
U.S. Pat. No. 4,105,547 discloses a cross-flow filtration device using an auxiliary longitudinal pressure drop compensation system. This is achieved by arranging for the outer surface of the support at the permeate side to be swept by permeate circulating in the same direction as the liquid to be treated so as to thereby set up a longitudinal pressure drop in the permeate chamber so that the transmembrane pressure remains approximately constant along the filter.
EP-A-0,333,753 discloses an embodiment of this device making it possible to compensate the transverse pressure loss variation set up by the circulation of a liquid inside one or several channels. Like the previous device, it is provided for permeate to circulate at the outer surface of a tubular membrane, of a porous support having a channel or, of a porous block also having one or several channels. The filtering media can be assembled into a one-piece structure or a bundle inside a housing in which the permeate chamber is filled with filling bodies such as ball or pellets which set up resistance to longitudinal flow of the permeate suitable for counter-balancing the longitudinal pressure loss caused by the liquid to be treated circulating in the channel(s) covered with a filtering layer.
These two prior art systems require a permeate recirculation loop, driven by a circulation pump, to be set up, the latter being required to provide the desired pressure head. Such systems of necessity employ specific casings or enclosures in which a permeate circulation can be set up at the outer surface of the filtering media and in the same direction as the circulation of the liquid to be treated inside the channel(s).
These prior art devices suffer from several disadvantages, such as:
cost overhead of providing the recirculation loop and its control and regulation system;
energy costs associated with the operation of this additional loop;
supplementary costs associated with the specific nature of the outer casing(s).
The invention thus sets out to provide a cross-flow filtering device which is simple, requires no adaptation of existing equipment, and involves no additional energy costs.
Thus, the invention provides a macroporous support for cross-flow filtration, said support having a permeability gradient in the direction of flow of the fluid to be treated.
The invention also provides a macroporous support for cross-flow filtration having a mean porosity gradient at a belt region in the direction of flow of the fluid to be treated, mean porosity increasing in said direction of flow.
According to one embodiment of the macroporous support, the mean porosity gradient corresponds to an impregnation gradient starting from the outer surface of said support.
According to a further embodiment of the macroporous support, the diameter of the pores in the impregnated region is comprised between 0.1 and 0.8 times the pore diameter in the non-impregnated region, preferably between 0.3 and 0.5.
According to one embodiment of the macroporous support, the ratio between outlet mean porosity and inlet mean porosity is comprised between 1.1 and 4.
According to another embodiment of the macroporous support, the level of initial porosity is comprised between 15 and 45%.
The invention also provides a membrane comprising the above macroporous support in association with a filtering layer.
The invention also provides a method for preparing the macroporous support comprising the step of immersing an initial macroporous support with its lower end closed, in a slurry or in an organic solution in a substantially vertical position.
According to one embodiment of the method, the dwell time is comprised between 0 and 15 sec., preferably between 0.5 and 8 sec. for those parts which are respectively least and most immersed.
One embodiment of the method for preparing a macroporous support comprises the step of spraying a slurry or an organic solution onto a normal macroporous support, the spraying region being moved along said support.
According to one embodiment of the method, the speed of movement of a spraying nozzle is comprised between 0.1 cm/s and 3 cm/s, preferably 0.7 cm/s and 1.7 cm/s.
According to another embodiment of the method, the throughput of sprayed material is variable along the support, the speed of displacement of the spraying nozzle being constant.
According to one embodiment of the method, the throughput of sprayed material is constant and the speed of movement of the spraying nozzle varies along said support.
In one embodiment, the method of preparing the macroporous support comprises the step of saturating a normal macroporous support with water and then the step of injecting gas into said support, the outlet end of which is free, partially closed off or completely closed off, and the step of immersing, in a substantially horizontal position, the support in a slurry or an organic solution.
The invention also provides a macroporous support saturated with liquid, having a gradient of its free volume.
The invention also covers the use of a macroporous support as defined above for cross-flow filtration.
The invention also covers the use of a membrane as defined above for cross-flow filtration.